Sergey, I understand that, but I don't care. The only people I've ever seen _use_ this are people writing an entirely different shell interface. They're rare. There's no value in complicating doctest to cater to theoretical use cases that don't exist - to the contrary, that's counterproductive complication.
As your IPython example showed, their job is harder than _just_ replacing PS1/PS2. See their `parse()` function. They want to cater both to IPython's PS1/PS2 conventions _and_ to the standard CPython terminal shell PS1/PS2 conventions.
But they don't cater to _more_ than just those either. Why should they? Same thing here: extra complication for no good reason.